Harmful materials and pathogens found in wastewater fluids, including wastewater, wastewater sludge and waste activates sludge can present a significant risk to the environment and to human health if left untreated. Accordingly, various organic, inorganic, chemical, and microbial components of wastewater fluids must be treated before waste products may be discharged to the environment. Examples of such wastewater fluids include industrial waste sludge, municipal wastewater, chemical processing effluent, paper mill effluent, livestock waste, etc. Treatment of these wastewater fluids is usually carried out in Waste-Water Treatment Plants (WWTPs).
Referring to FIG. 1, a schematic view of treating municipal wastewater fluid in a prior art wastewater treatment plant (WWTP) 100 is illustrated. Wastewater fluid can flow first into a preliminary treatment station 102. The preliminary treatment station 102 may include one or more screens (not shown), which can, for example, be large metal grates that prevent larger objects (trash, grit, sand, etc) in the wastewater fluid stream from passing further downstream.
After the wastewater fluid stream passes through the preliminary treatment station 102, the wastewater influent enters a primary settling clarifier 103 of the WWTP 100 where raw sludge (also referred to as primary sludge (PS)) is separated from the wastewater via flocculation, sedimentation, and other primary settling techniques.
The remaining wastewater fraction (separated from the primary sludge) that is discharged from the primary clarifier 103 still contains a relatively high concentration of suspended bio-solids and dissolved bio-waste, nitrates, phosphates, etc. This wastewater fraction, which is also referred to as primary effluent, is directed to one or more aeration tanks 104, where aerobic microorganisms treat the wastewater in the presence of air that is pumped into the aeration tank 104 to produce an aerated wastewater effluent.
It should be noted that some WWTPs forgo the stages of treatment on the preliminary treatment station 102, and on the primary settling clarifier 103, and the wastewater fluid stream in its entirety is transferred to one or more aeration tanks 104. The action of bacteria in the aeration tank(s) 104 is to reduce the phosphates, nitrates and dissolved or suspended bio-waste.
The aerated wastewater effluent leaving the aeration tank 104 is referred to as activated sludge (AS). The AS is transferred to a secondary settling clarifier 105, where further settling can occur. The wastewater fluid leaving the secondary clarifier 105 has two fractions, such as a fraction containing a higher percentage of bio-solids (microbial matter), and a fraction containing a lower percentage of bio-solids.
The wastewater fluid fraction leaving the secondary clarifier 105 and containing a higher percentage of bio-solids is referred to as waste activated sludge (WAS) or as secondary sludge. Some of the secondary sludge is usually returned to the aeration tank 104 to help perpetuate the aerobic biodegradation process. This secondary sludge is referred to as return activated sludge (RAS).
The WAS from the secondary clarifier 105 and the primary sludge (raw sludge) from the primary clarifier 103 are transported to an anaerobic digester 106.
When desired, the waste activated sludge from the secondary clarifier 105 and the primary sludge from the primary clarifier 103 may be passed through primary and secondary thickeners 107 and 108, correspondingly, where access water can be removed from WAS to increase the solid content. The access water can, for example, be removed by adding chemicals, such as polymers in combination with trivalent iron or lime. Likewise, such actions such as straining, floating, or gravity settling can also be used for removing access water.
The thickened primary sludge and the thickened waste activated sludge may be passed along to the anaerobic digester 106 for about 15-21 days. In the anaerobic digester 106, the primary sludge and waste activated sludge are exposed to microorganisms in an oxygen-poor environment for anaerobic digestion that further degrade the sludge biologically by subjecting them to anaerobic fermentation to yield by-product gases, such as methane (CH4), carbon dioxide (CO2), hydrogen sulfide (H2S) and ammonia (NH3).
At least two product streams may exit the anaerobic digester 106. A first product stream contains by-product gases CH4, CO2, H2S and NH3, and a second stream contains digested sludge which is also referred to as digestate, that contains digested solids, microbiological processors, and also liquid fraction. It should be noted that although methane may represent energy resources and may be gathered and used, other by-product gases emit bad odors, cause pollution, and are corrosives.
The digestate is transferred to a dewatering system 109 where the sludge exiting the anaerobic digester 106 is subjected to pressure for dewatering to further separate liquids from the bio-solids to create a “dry” solid material, in the form of a sludge cake or a sludge powder. The “dry” solid material can be carted away as expellant, while the liquid fraction may be reclaimed and returned to the aeration tank 104.
The dewatering system may include filter press 109a, belt press 109b or centrifuge 109c. Moreover, adding chemicals, such as polymers and flocculants is usually used to aid the dewatering process.
Due to the physical nature of the digestate being composed of spongy and/or closed cells, The water content in the “dry” expellant after the conventional dewatering treatment can still be around 80 weight percent or even higher, resulting in a large expellant volume. The expellant also has a high risk of containing harmful pathogens and parasites, so it must be made inert by boiling, burning, or composting to high acidity.
The expellant from the WWPT is evacuated and transported to an incinerator plant or a composting farm (not shown). Incineration is usually difficult due to high water content. It requires a large amount of fuel and leave ashes that still must be disposed of. Moreover, incineration produces high carbon emissions.
At a composting farm, it usually takes several weeks for the sludge cake to become inert and safe for use as a fertilizer or for its disposal in a landfill.
It is known in the art that the majority of the water contained in municipal waste-water sludge is “bound” water which is contained within and between molecular cells. Molecular cells in the sludge can be presented in waste-water sludge as individual cellular units or as cellular units assembled in flocs. The water molecules contained within the cell, for purposes of this application, are referred to as “intra-cellular” water molecules, while the water molecules between the cells and bound thereat via both mechanical and electrical bonding, are referred to as “intercellular” water molecules. When sludge exiting the anaerobic digester 106 is treated at a municipal treatment plant by conventional dewatering belt presses and/or centrifuge methods, intra-cellular and intercellular water is not completely released.
Techniques for dewatering and reduction of volume and weight of WAS sludge are described in U.S. Pat. Nos. 6,491,820; 6,540,919; 6,709,594 and 7,001,520. These references describe systems and methods for treatment of biologically-active waste-water sludge by a pulsed electric-field system which applies non-arcing high voltages to sludge. The pulsed electric-field provides electroporation of the sludge, causing disruption of the cellular structure and breaking down intra-cellular and intercellular molecular bonds of WAS sludge. As a result, intra-cellular and intercellular water is released from the WAS sludge. After releasing the intra-cellular and intercellular water, the organic solid contents, suspended in solution, are reduced in volume and mass, which can simplify sludge post-treatment processes. Moreover, the electroporated sludge can be directed to bioreactors, such as aerobic, anoxic or anaerobic, for performing biological digestion, where the electroporated sludge can be used as food for the microorganisms participating in the biological digestion.
The wastewater fluid fraction leaving the secondary clarifier 105 and containing a lower percentage of bio-solids is referred to as secondary wastewater effluent. The secondary wastewater effluent can be heavily polluted and contain pathogenic bacteria and viruses. In order to provide subsequent purification, the secondary effluent is transported to a final treatment station 120. The final treatment station 120 may, for example, include chemical disinfection by chlorine, hydrogen peroxide, ozone etc. Likewise, it may involve the use of ultra-violet (UV) light to destroy the pathogens. These disinfection processes may be concurrent or consecutive.
The resultant flow may then be directed to the filtration station 121. The filtration station 121 is an optional station, and may be included or omitted depending upon the use for which the resultant treated water is intended. One or more filters, such as sand or crushed coal filters, may be used to remove impurities remaining in the treated water stream. Bio-solids collected on the filters may be removed, for example, by backwashing the filters, and directed to the anaerobic digester 106. The resulting water stream, that is referred to as tertiary effluent, may be discharged into a river, lake or ocean, or put to an alternative use, such as for irrigation or for industrial processes.
The disinfection of wastewater effluent has been historically accomplished through the addition of chlorine compounds. There are major health and safety concerns associated with handling chlorine compounds. In recent years, there have been increased concerns that chlorine can combine with organic material in the effluent to produce chlorinated organics, which are both toxic and potentially carcinogenic. Although some efforts are being made to substitute less toxic chlorine compounds, there is an industry-wide trend towards phasing out the use of chlorine as a disinfectant agent.
Other disinfection technologies employed in wastewater treatment, that involve the use of ultra-violet (UV) light or ozone, are relatively costly. The effect of these techniques is short-lived, so that pathogen re-growth can occur, compared to longer-lasting chlorines. In the case of a UV process, the capital costs include the construction of the flow-through mechanism, and the multiple UV bulbs (lamps) that are required. The operating costs include power, timely replacement of bulbs, and regular cleaning of the bulbs. Major costs for disinfecting with ozone include the ozone generator and commercial oxygen, which is used as the feed source. When air is used as the feed source, the size of the ozone generator must be approximately doubled, therefore doubling the capital cost.
A technique known in the art, usually under the name “electro-hydraulics”, utilizes arcing high-energy electrical discharge into a volume of liquid or slurries or other fluid for the purpose of disinfecting, changing chemical constituents and recovering metals and other substances from the fluids (see, for example, U.S. Pat. No. 3,366,564 to Allen; U.S. Pat. No. 3,402,120 to Allen et al.; and U.S. Pat. No. 4,957,606 to Juvan).
According to this technique, an electro-hydraulic shock wave within the liquid or slurries, intensive light radiation and thermo-chemical reactions are initiated by arc discharge into a spark gap formed by the electrodes immersed in such fluids.
FIG. 2 shows an electric scheme of a typical prior art system 10 for treatment of wastewater sludge or other contaminated fluid by utilizing high-energy arcing electrical discharge. The apparatus 10 includes a high-voltage supply device 11 having a rectifier (not shown) coupled to a high voltage capacitor bank 12 that comprises one or more capacitors. The coupling of high-voltage supply device 11 to the capacitor bank 12 can, for example, be a direct “galvanic” connection.
Alternatively, as is explained below, the connection can be through a resistive element and/or a switching element. The supply device 11 and the high voltage capacitor bank 12 form together a charge circuit A.
The system 10 also includes a high current switch 13 in series with the capacitor bank 12 and a pair of electrodes 14a and 14b separated by a gap in series with high current switch 13. In operation, the electrodes 14a and 14b are immersed in a liquid 15 for providing an electric discharge in the gap therebetween within the liquid. The capacitor bank 12, together with the high current switch 13, the electrodes 14a and 14b, and all interconnection cables therebetween form a discharge circuit B. For safety reasons, one of the terminals of the high-voltage supply device 11 (for example, which is connected to the electrode 14b) is permanently grounded. When desired, only one of the electrodes (14a in FIG. 1) is immersed in the liquid 16 under treatment, whereas the second electrode (14b in FIG. 1) can be coupled to or associated with a conductive body of the treatment vessel 16 holding the liquid 15.
In operation, the capacitor bank 12 is charged by the voltage supply device 11. During charging of the capacitor bank 12, the high current switch 13 is open. After charging, the capacitor bank 12 can be discharged by closing the switch 13, in order to supply a high voltage to the electrodes 14a and 14b, and thereby generate an electric current pulse therebetween through the liquid under treatment. The closing of the high current switch 13 is usually activated by an ignition circuit (not shown) launching an ignition electric pulse to the switch 13.
Despite its apparent simplicity, the system 10 suffers from a number of limitations. In particular, the current charging the capacitor bank 12 has the form of an attenuated exponent. Accordingly, the charging current is high only at the very beginning of the charging process, and then the charging current decreases over time. As a result, the power supply efficiency is low.
Another drawback is associated with the fact that the submerged electrodes 14a and 14b are subjected to damage from the pressure wave and to the electrical erosion produced by the arcing current. Thus, the electrodes 14a and 14b must be either massive or frequently replaced.
Moreover, a large portion of the discharge current is lost in ionizing the liquid before any arcing can occur. Thus, with massive, robust electrodes having large surface areas, the loss in ionizing can consume nearly all of the stored capacitor energy, resulting in generation of only a week arc, or no arc at all, thus making the hydraulic shock insufficient for the desired purpose.
Another drawback is associated with transient current behavior in the discharge circuit B. Since the discharge circuit B represents a series RLC circuit, the transient response of the circuit B depends on the damping factor ζ that is given by
      ζ    -                  R        2            ⁢                        C          L                      ,where C is the capacitance (in Farads) of the capacitor bank 12, L is the inductance (in Henrys) and R is the resistance (in Ohms) of the discharge circuit B.
The current behavior i(t) during a transient response for different ζ is shown in FIG. 3A. As can be seen, this behavior depends on the value of ζ. In particular, when ζ<1 (the under-damped response, curves 21-23), the transient current decays with oscillation. On the other hand, transient current decays without oscillations occur when the ζ≥1 (the critically damped response, shown as curve 24, and over-damped response, shown as curve 25).
High values of L prevent the current from rising fast, making a larger portion of the stored charge be lost before an arc forms. On the other hand, large values of R limit the value that the current may rise to, and thereby the power of the arc when it forms (an over-damped system response, with ζ>1, shown as curve 25, or critically damped response, shown as curve 24). However, low value of R may result in an under-damped response, with ζ<1, (curves 21-23), that produces polarity reversal in the discharge circuit B, as transient current decays with oscillation.
In the case of oscillating current decays, the negative reverse components IR of the oscillating transient current i(t) can either over-deplete and then reversely charge the capacitor bank 12, thereby producing a reverse voltage of high amplitude across the capacitor bank 12, or draw the corresponding reverse discharge current through the high-voltage supply device 11, thereby damaging it.
In order to decrease the reverse current of the electric discharge through the high-voltage supply device 11, a current limiting resistor 17 is usually included into this chain between the capacitor bank 12 and the voltage supply device 11 for limiting the discharge current drawn by reversed polarity during discharge. Although this provision enables protection of the voltage supply device 11 from damage, it results in electric losses in the resistor 17, reduced charging current, reduced efficiency and extra expenses.